Concept

Laporte's rule — where it appears

The parity selection rule: in a centrosymmetric molecule an electric dipole transition between two states of the same parity has a vanishing moment. It is why d–d bands are weak, and why removing the centre of symmetry makes them stronger.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Dipole selection rules in Td. For every pair of symmetry species, whether an electric dipole transition between them is allowed and along which polarisation. An entry is allowed exactly when the triple product of representations contains the totally symmetric one.

Selection rules are one theorem

An integral over all space vanishes unless the integrand is totally symmetric. Every selection rule in spectroscopy is that sentence with a different integrand — and the rule of mutual exclusion falls out rather than being remembered.

symmetry · Representation

Dipole selection rules in Oh. For every pair of symmetry species, whether an electric dipole transition between them is allowed and along which polarisation. An entry is allowed exactly when the triple product of representations contains the totally symmetric one.

Why a d–d band is weak

In a centrosymmetric complex the transition between the two halves of a split d shell is forbidden — exactly, by parity, with no small quantity anywhere. What makes it visible at all is that the molecule is never quite centrosymmetric, and the computation says which vibrations do the work.

spectra · Representation

Named alongside it

The objects these essays reach for when they reach for this one.

Parity (g and u)Selection rules Transition moment Vibrational modes Absorption Character table D d transition Dipole moment Irreducible representations Ligand field The rule of mutual exclusion Point group

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